Visbreaking is a mild cracking operation used to reduce the viscosity of heavy residua. The heavy residua are sometimes blended with valuable lighter oil, or cutter stocks, to produce fuel oils of acceptable viscosity. By use of visbreakers, the viscosity of the heavy residua is reduced so as to reduce the requirement of the cutter stock. The ultimate objective of the visbreaking operation is to completely eliminate the need for cutter stocks.
Sometimes visbreakers are also used to generate more gas oils for catalytic cracking and naphtha for reforming to increase the gasoline yield in the overall refining operation. To achieve these goals, the visbreaker has to be operated at high enough severity to generate sufficient quantities of lighter products.
The visbreaker feedstock usually consists of a mixture of two or more refinery streams derived from sources such as atmospheric residuum, vacuum residuum, furfural-extract, propane-deasphalted tar and catalytic cracker bottoms.
Most of these feedstock components, except the heavy aromatic oils, behave independently in the visbreaking operation. Consequently, the severity of operation for a mixed feed is limited greatly by the least stable (highest insolubles formation) components. As a result, the potential for light product generation of the more desirable components can not be realized.
Accordingly, it is an object of this invention to provide a process for improving the performance of visbreakers for the production of an increased yield of light products such as heating oil and gas oil.
It is another object of this invention to provide a process for the production of a visbroken hydrocarbon product which meets heating oil viscosity specifications with little or no blending with external cutter stocks.
It is a further object of this invention to provide an improved visbreaking process for the production of two or more visbroken effluents which are stable and compatible and which can be blended together and/or with other fuel range stocks without formation of sludge or precipitate.
Other objects and advantages of the present invention will become apparent from the accompanying description and Example.